Transmission system



IOO

Dec. 19, 1944.

G. M. GIANNINI ET AL 2,365,187

TRANSMISSION SYSTEM Filed Nov. 14, 1941 5 Sheets-Sheet 1 FIG. I

SIGNAL CURRENT AMPLIFIER CONTROL BALI HYBRID NETWORK I NETWK SYSTEMSIGNAL CURRENT AMPLIFIER EAST STATION I O GABRIEL M IZITIKII II BY BENEISENBERG MAMI W m ATTORHEL 1944 e. M. GlANNlNl ET AL TRANSMISSIONSYSTEM 5 Sheets-Sheet 2 Filed Nov. 14, 1941 CENTRAL REPE'ATER I2 II "II-moakmz 4/ A INVE M. 6!

NTORS GABRIEL ANNINI BY BEN EISENBERG Mxw, M xim ATTORNEYS G. M.GIANNINI ET AL 2,365,187

TRANSMISSION SYSTEM 5 sheets Sheet 5 Filed NOV. 14 1941 WEST STATION Huov mm N mN% w V B mM 6 EE M A M GB Yf BM 3 Cu F ATTORNEYS 19,1944. G.M. GIANNINI Em 2 365,187

TRANSMI SS IC JN SYSTEM Filed Nov. 14, 1941 5 Sheets-Sheet 4 INVENTORSGABRIEL M. GIANNINI BY BEN EISENBERG ATTORNEYS 19 e. M. GlANNlNi ETAL2,365,137

TRANSMISSION SYSTEM Filed Nov. 14, 1941 s Sheets-Sheet 5 lun 5 .2.

' INVENTORS GABRIEL M. GIANNINI BY BENEISENBERG ATTORNEYS Patented Dec.19, 1944 m orncE -TRANSMISSION SYSTEMY Gabriel M.'Giannini, NorthHollywood, Calif., and

Ben Eisenberg, Brooklyn, N.'Y., assignors, by mesne assignments, toAutomatic Electric Laboratories, Inc a corporation of DelawareApplication November 14, 1941, Serial No. 419,164 14 clai s. (01.1715170) I The present invention relates to signal current transmissionsystems and, more particularly,- to improvements in voice operatedcontrol circuits of the character utilized in conjunction with two--channel transmission systems to prevent the combined signal currentgains of the two coupled channels from exceeding the combined lossesthrough the channels and through the couplings between the channels.

It is an object of the present invention to pro; ,vide a signal operatedcontrol circuit of the characterdescribed which is simple in arrangementand is positive and reliable in'its operation to increase the signalcurrent gain through the channel in use and concurrently to decrease thegain through the inactive channel.

According to another object of the invention, an improved arrangement isprovided for preventing the control circuit from altering the gain ofeither of the two channels until the signal input to the channel in useexc'eedsa predeter- -mined value.

It is another and more specific object of the invention to provide animproved signal operated control circuit of the character describedwhere in the space current path of a control tube, which is controlledin accordance with the signal input to one of the channels, is connectedin circuit with the cathode of one of the amplifier tubes in the otherchannel, so that as the space current re-- sistan'ce of the control tubeis varied under the influence of signal currents traversing the onechannel, the heating current of the amplifier tube cathode is changed todecrease the signal current 'gain through the other channel.

According to another object of theinvention,

.the space current path of the control tube is also connected tocontrolthe shunt impedance ,includes the features of the inventionbriefly referred to above; Fig. 4a illustrates one improvedloudspeaker-microphone arrangement which may be usedat each of thestations of the system;

Figs. 4b, 4c and 4d illustrate amodified arrangement of theloudspeaker-microphone unit; Fig. 4e illustrates another modification ofthe loudspeaker-microphone unit; and Figs. 5 to 9, inclusive,illustratethe details of the transmitting and receiving elementsprovided at'each of the suband a'west station l l interconnected by apair oi lines l3 and HI having a central-repeater I2 connectedtherebetween. This repeater may be pro- ,vided at a central point in thesystem and may be used to stabilize, the transmission level over the twoconnected lines l3 and 14. Briefly described, this repeater comprises.an veast-west amplifier 201, a, west-east amplifier 200, a pair ofhybrid systems 202 and;203, balancing networks 204 and 20-5 individualto the hybrid coils and respectively operative to balance the lineimpedances of the lines l3 and J4, and a signal operated control network220which is operative to control the gain .of the twqamplifiers 200 andMI in accordance with the direction of signal current transmission overthe lines 13 and I4.

Ell/lore specifically considered, the west-east amplifier 200 comprisesa coupling transformer 206 having its input Winding connected to thesignal transmitting terminals of the hybrid system 203, and its outputor secondary winding coupled to the input electrodes 20112 and 2010 ofthe threeelectr d amplifier tube 201. The output electrodesflola and20lcof this tube are coupled to the, signal input terminals of the hybridsystem 202 through a coupling transformer 208. The

1 opposite or east-west amplifier 20] similarly comprises a couplingtransformer 2 having its prii ting terminals 'of thehybrid system 202,and its secondary windingcoupled to the input electrodesmary-windingjconnected to the signal transmit- 2l2b and2l2c.,,of-the'three-electrode amplifier tube 2l2.- The output electrodesHM and 2I2c ofthis tube are coupled to the'signal input terminals ofthehybrid system 203 through a cow pling transformerZ l3. Preferably,the amplifier tubes 20'! and 2l2 are of the well-known 6C5 type,utilizing indirectly heated cathodes and characterized by acomparatively high amplifi- -.cation' factor.

The. control network 220, asindicated above, is

I provided for the purpose" of increasing the gain ofthesignalamplifier200or 20Lwhich. is in use: during signal current transmissionoverthe tubes 232 and'233 in opposite senses.

two lines I3 and I4, and for concurrently efiecting a correspondingdecrease in the gain through of the east-west channel amplifier tube 201over a signal current path which includes the couplin transformer 228,the three-electrode amplifier tube,

221, the adjustable voltage dividing resistor 226 and the couplingtransformer 225. The. duplex diode 23I functions to rectify signalcurrents appearing across the secondary windings of the couplingtransformers 224 and 228, respectively, and to impress the rectifiedvoltagesjupon the input electrodes of the two direct current amplifierThus, it will be noted that the diode section23Ia, 23Ic is included in aclosed direct current circuit which includes the space current pathbetween these electrodes, a resistor 230, the secondary winding of thetransformer 224, and a resistor 234. The other two electrodes 23Id, 23Ie of the duplex diode 23I are similarly included in a circuit whichincludes the resistor 234, the secondary winding of the transformer. 228and a resistor 229. A smoothing condenser 236 is connected in shunt withthe resistor 234 in order to smooth out alternating components ofthevoltage developed across this resistor and to determine the timeconstant of the control circuit. The voltages developed across the twohalves 234a and 2341) of winding of the transformer 200. This winding isbridged by a terminating network which comprises the shunt-connectedresistor-209 and condenser 2I0. Similarly, the output circuit oftheother tube 233 includes a biasing resistor 23Gb which is shunted by asmoothing condenser 238 and is connected through the secondary windingof the coupling transformer 2 to the control grid 2l2b of the east-westamplifier tube 2I2. This secondary winding is also bridged by aterminating network which includes the shunt-connected resistor 2I3 andcondenser 2l4. Preferably, the control and amplifier tubes 223, 221, 232and 233 are of the commercial type 6G5, and the duplex diode tube 23I isof the 61-16 type.

Anode voltages are impressed upon anodes 201a and 2I2a of the two signalamplifier tubes 20'! and 2I2 from the voltage source 2, over paths whichrespectively include the signal current choke coils 245 and 2I5. These"coils and the source 24I are by-pa-ssed forsignal currents by .thecondensers 244 and 2I6. In this regard it will be noted that thecathodes H20 and 2010 of the two amplifier tubes are connected to thetapped point along the voltage dividing resistor 242 in order to providethe required potential on these cathodes. The voltage of the source 24Iis also positively applied to the anodes of the two tubes 223 and 221through the primary windings of the coupling transformers 2-24 and 22,8,respectively. The direct voltage across the section 242a of the resistor242 is also positively applied to the anodes of the two control tubes232 and 233 through the resistors 235a and 23612, respectively. Thecathodes of the two tubes 223 and 221 are positively biased with respectto their associated control electrodes by the voltage drop across thevariable section 239a of the voltage divider 239, which divider isbridged across the section 242a-of the resistor 242.

Referring now more particularly to the equipment provided at the eaststation I0, this equipment is illustrated in Fig. l of the drawings ascomprising a transmitting element in the form of a microphone I00,areceiving element in the form of a loudspeaker IOI, a hybrid system I02,and a balancing network I03 which is provided to balance the impedanceof the line I3 so that signal energy transfer between the transmittingand receiving channels of the station through the hybrid system isreduced to a minimum. 0 The station equipment further comprises atransmitting amplifier I04, areceiving amplifier I05, and a controlnetwork I06, which functions to-control the signal current gain throughthe two amplifiers in the manner more fully pointed out hereinafter.

In general, the arrangement of the equipment provided at the weststation II is identical with that provided at the east station 10 and,accordingly, only the details of the station II have been illustrated inthe drawings. Briefly described, the equipment provided at the weststation II includes one of the three loudspeaker- .microphone unitsillustrated in Fig. 4 oi the drawings, and the circuit equipmentillustrated in Fig. 3 of the'drawings. This circuit equipment comprisesa transmitting amplifier 300 and a receiving amplifier -30l which arerespectively coupled to the "transmitting and receiving terminals of ahybrid system 302. A balancing network 303 of the adjustable resistivetype is provided for balancing the impedance of the line I4 tominimizeenergy transfer between the transmitting and receiving channelsof the station II. For the purpose of controlling the gain of the twoamplifiers 300 and 323i to preserve stability of the station circuit, acontrol network 3I5 is provided. The various anode and biasing voltagesrequired for operation of the amplifier tubes and the tubes of thecontrol network 3I5 are derived from a power pack indicated generally at335.

More specifically considered, the transmitting amplifier 300 comprises athermionic tube 306 of the well-known pentode type,' having its inputelectrodes 30Gb and 306c cou-pled to one of the three microphones 40I, MI and 42I over a channel which includes an adjustable voltage dividingresistor 305, a condenser 363, a coupling transformer304 and the cordconductors 430. The output electrodes of this tube are directly coupledto the input win-ding of the hybrid system 302, which is shunted by aresistor 338. The other signal amplifier 30I similarly comprises athermionic tube 308 of the well-known pentode type, having'itsinputelectrodes 300p and 300c'coupled 2,365,187 to the receivingterminals of the hybrid system.

302 through an adjustable voltage dividing resistor 301. The outputelectrodes of this tube may be coupled to the loudspeaker of any one ofthe three translating units 400, 4 I and 420 in use, over a channelwhich includes the coupling transformer 309 and the cord conductors 43!.The two tubes 306 and 308 are provided with cathodes 3060 and 3030 whichare of the filamentary type, such that rapid changes in the electronemission of the cathodes is obtained in response to changes in thecurrent traversing the cathodes.

The input and output terminals of the control network 3! 5 are coupledto the receiving channel of the station circuitv at a point justfollowing the signal amplifier tube 308. Briefly described, this networkcomprises a signal amplifier tube 322 having'its input electrodes 3221)and 3220 coupled to the output circuit of the amplifier '30! over achannel which includes thefrequency'discriminating circuit 3H5. Thiscircuit, which comprises a coupling condenser 3! 1, a loading condenser320, a voltage dividing resistor 3! 8 and two resistors 3!9 and 32!, isdesigned to favor frequencies lying within the lower portidn of thevoice frequency range, and to discriminate against frequencies in theupper portion of this range. The output electrodes of the signalamplifier tube 322 are coupled through a coupling transformer 324 to theelectrodes of a triode 326 which is connected to operate as a dioderectifier. A load resistor 328 shunted bya signal current by-passcondenser 329 is connected in circuit with the space current path of thetube 326, and the voltage developed thereacross is utilized to controlthe potential difference between the input electrodes 33012 and 3300 ofthe control tube 330. In order to prevent the control circuit 3l5 fromeffecting a change in the gain of either of the two channel amplifiertubes 306 and 308 when the signal input to the station I! is below apredetermined value, a biasing battery 325 is provided in circuit withthe tube '326. This battery functions to prevent the tube 326 frompassing current until the signal input to the'station ex-.

ceeds the selected predetermined value.

The control tube 330 is provided with a space current path between itsoutput electrodes 330a and 330s which may be connected in parallel withthe space current path of the signal amplifier tube 308, and in serieswith a cathode biasing resistor 352. It will be apparentthat with thespace current path of the tube 330 shunting the output circuit of thamplifier tube 308,, the signal current gain through the receivingamplifier 30! is varied directly in accordance with the resistance ofthe space current path through the tube'330. It will also be noted thatthe cathode biasing resistor 352 is shunted by the cathode 3060 of thetransmitting amplifier tube 306, whereby the electron emission of thetube 306 is controlled in accordance with the current traversing thespac current path of the tube 330. This cathode is shunted by a largecondenser 365 which functions to by-pass the cathode for signalfrequency currents.

Briefly considered, the power pack 335 comprises a voltage transformer336 having a primary winding 33! which is arranged-to be connected to asource of commercial frequency current through a manually operableswitch 338 and is provided with a high voltage secondary winding 339, alow voltage cathodeheating winding 340, and a low voltage cathodeheating winding 34!.

fier tube342 having'its two anodesconnectedto the outer terminals ofthe'high voltage windin 339, and its cathode connected to a load-'resistor 349" through a filter network which comprises a series chock346, a pair of shunt connected filter.

' condensers 34'! and 348, and a bleeder resistor vide bias voltages forthe two tubes 306 and 330.

Thus the voltage of approximately 5. 5 volts which is developed acrossthe section 343a of the resistor 343 during operation of the apparatus,is'nega-I tively applied'to the control electrode 30Gb of the tube 306over a path which includes the filter resistor 364 and the lowersection. of the voltage dividing or volume control'resistor 305.Thissection of the resistor 343 is shunted by an alternating currentby-pass condenser 344 to prevent noise from being introduced into thesignal amplifier 300 from the power pack. The Voltage of approximately18.5 volts which is developed across the section 34312 of the resistor343 during operation of the power pack 335, is negatively applied to thecontrol electrode 33% of the tube. 330 over a path'which includes-theresistor 328. This section of the resistor 3'43 and the resistor 328 arebypassed for alternating currents by the condensers 33! and. 345. Theentire voltage of approximately 270 volts developed across the loadresistor 349 during operation of the power pack'335, is positivelyapplied to the anode 308a of the receiving amplifier tube 308 over apath which includes the resistor 362 and the primary winding of thecoupling transformer 309. Thisvoltage is also positively applied to thescreen electrode 308d of the tube 308 over a path including the resistor362. Depending upon the position of an On-Offswitch 36!, which isprovided for rendering the control network 3l5 active or inactive asdesired, this",

through the resistor 362 and the primary winding of the transformer 309.

A portion of the voltage developed across'the load resistor 349, i. e.,approximately 110 volts, is 'positively applied to the anode of thesignal amplifier tube 322 through the primary winding of the couplingtransformer 324, and tothe anode of the transmitting amplifier tube 306over a path which includes the re-, sistor 366 and the signal inputwinding of the hybrid system 302 in parallel. This voltage is alsodirectly applied to the screen electrode 306d of the transmittingamplifier tube 306. A bypass condenser 32'! is connected in shunt withthe portion of the resistor 349 from which the 110- volt anodepotentials are derived, in order to preventthe alternating components ofthis voltage from being impressed upon the screen electrode 306d and theanodes 306a and 322a.

Cathode heating current is supplied to the cathode 3080 of the receivingamplifier tube 308 from the low voltage winding 34! of the transformer336, it being noted in this regard that the mid-point of this winding isconnected to ground over a path which includes the cathodebiasingresistor 35! and the biasing resistor 352 in series, whereby the cathode308a is normally maintained at a potential which is substantiallypositive with respect to its associated control grid 30%. This biasingpath is by-passed for alternating currents by a shunt connectedcondenser 353. Cathode The power pack also includes a full waverectiheating cur n are up to the'cathodes of 331 of the transformer 336.More specifically, the

low voltage winding 35B is connected to supply heating current to thecathode 3260 of the tube 326 and is provided with a center tap which isconnected to one terminal. of the load resistor 328. The low voltagewinding 35'! is connected to'supply heatingcurrent to the cathode 3220of the tube 322 and is provided with a tapped center point which isconnected to ground through a'cathodebiasing network which comprises aresistor 359 shunted by a signal current by-pass condenser .360. Thethird low voltage winding 358 is connected to supplyv cathode heatingcurrent to the cathode 330c of the tube 330 and is provided with atapped center point which is connected to ground through the cathodebiasing resistor 352.

Asindicated above, any one of the three loudspeaker-microphonearrangements illustrated in Figsmla, 4b, 4c, 4d and 4e of the drawingsmay be used in conjunction with the circuit equipment shown in Fig. 3 ofthe drawings. The common purpose of the three arrangements is tominimize acoustical coupling between the loudspeaker and the microphoneand thus reduce the tendency of the station circuit to oscillate andproduce singing or howling. Briefly considered, the unit illustrated inFig. 4a of the drawings comprises a microphone 48! which is arranged tobe primarily responsive to sound waves transmitted thereto along ahorizontal path, anda loudspeaker 402 which points upward and isarranged to transmit sound waves in a substantially vertical direction.These two translating devices are housed in a casing 4000. which isprovided with curved outer surfaces, and in order to minimize conductiveor mechanical coupling 'between the two translating elements 40! and402, the casing 400a is filled with acoustical damping material, such,for ex- 4 l a, and are surrounded by felt or other acoustical insulatingmaterial to minimize the conductive coupling therebetween. In thearrangement illustrated in Figs. 4b, 4c and 4d, the sound chamberadjacent the diaphragm of the loudspeaker 422 communicatesiwith two setsof sound passages 424a and 424b which extend to the air surrounding theunit. These passages, at their outer ends, are flared away from eachother so that the sound emitted therefrom is directed away from the zoneintermediate the two sets of sound transmission openings. Thetransmitter elernent or microphone 42l is' disposed between the two sets.of sound passages 424a and 424i) and is provided with a diaphragm whichcommunicates with the air surrounding the unit through the elongatedsound passages 423. As best shown in ,Fig. 4d

slightly downward to prevent sound reflected from the walls and ceilingof the. room in which the unit is located from being transmitted to thediaphragm of the microphone 42!. The loudspeaker 422 and the microphone421 are housed within a casing 420a which is filled with acousticalinsulating material, such as felt, to minimize the conductive couplingbetween the two elements. If desired, the insulating material providedwithin the casing 420a may be compressed or molded, and the soundpassages 423, 424a and 424b may either be preformed therein during themolding process or may be cutin the formed block after the molding oithe structure is completed. As an alternative to this arrangement thetwo elements 42! and 422, each including its own sound directingpassages, may be constructed as separate units, and the two units maythen be housed within the casing 4200.. In the latter case a suitableamount of acoustical insulating material should be provided around eachof the two units in order to minimize the conductive couplingtherebetween.

In the operation of the loudspeaker-microphone arrangement illustratedin Figs. 4b, 4c and 4d of the drawings, sound waves developed duringoperation of the loudspeaker 422 and trans-. mitted through the soundpassages 424a and 424b, diverge at the openings of these passages andare transmitted away from the zone in which sound is transmitted to thesound passages 423 which communicate with the diaphragm of themicrophone 42!. By virtue of this arrangement a minimum of thereproduced sound energy is fed back through the passages 423 to the airchamber adjacent the microphone diaphragm.

Further to consider the three embodiments of the microphone-loudspeakerunits, it has been I found that station transmitter-receivercombinawords, signal currents having frequencies corresponding to theresponse characteristic peaks and reproduced by the receiver element,cause the maximum response of the associated transmitter element. It hasalso been observed that the acoustical coupling between the stationtransmitter and receiver elements is affected by the spacing between thesound transmitting and sound receiving openings which connect themovable diaphragms of the coupled elements with the surrounding air.More specifically, the coupling is greatest when the spacing between thesound transmitting and sound receiving openings of the two elements isof the order of one-half of the wave length representing the soundfrequency under observation. Accordingly, when the openings of the soundtransmitting and receiving elements are spaced apart by a distance equalto an odd number of quarter Wave lengths of a particularly objectionablefrequency, a minimum amount of acoustical coupling between the twoelements and, hence, a maximum of system stability, are obtained.Usually the most objectionable peak in the combined frequency responsecharacteristic of the transmitting and receiving elements occurs in thelower portion of the voice frequency range. For example, tests on aspecific combination of elements have indicated a maximum combinedresponse of the two elements at a frequency of the order of 600 cyclesof the drawings, thesecpassages are directed J5 Lper second, whichcorresponds to a wave length,

I of the opening and the center pole piece. magnetic circuit assembly iscompleted by means measured atordinary .room temperature, ofapproximately 22.3;inches. spacing between". the sound transmitting andre- In this case a mean ceiving openings of the loudspeaker and,microphone, respectively, of approximately 5.6 inches may be used toobtain the maximum of acoustical attenuation-between the two elements.Another factor which in part determines the acoustical coupling betweenthe two translating elements AM and 402, for example, is the size of thediaphragm used in the loudspeaker 402. In general,

it may-be statecl'that the smaller the loudspeaker diaphragm the lessthe acoustical coupling between the loudspeaker and its associatedmicrophone; .In other words, a loudspeaker um't having a small effectivediaphragm diameter is capable "of producing a greater output per unit ofdiaphragm'area, without producing instability of the station network.,More specifically, it has been found that if the effective diameter ofthe loudspeaker cone is made more than one-half the minimum wave lengthof the operating frequency range, the' acoustical coupling between theloudspeaker and the microphone elements becomes difficult to" control.In'the system under consideration a transmission band ranging from about350-cycles per second to approximately 33 cycles per sound is'utilizedfor voice current transmission'. Accordingly, a loudspeaker is providedwhich is equipped with a diaphragm having an effective diameter of lessthan 6 centimeters.

In brief, the assembly there'shown comprises a self-contained 'magneticcircuit which includes a permanently magnetized annular ring 500 formedof Alnico, a

bottom plate 50!, a center pole piece 502, and a top plate 503. Theparts 50!, 502 and 503 are preferably formed of Allegheny electric metalor other highly permeable magnetic material. The center pole piece 502is provided with a lower portion 502a of reduced diameten'which extendswithin a centrally disposed opening provided in the lower plate 50L Asweat connection between the side walls of .this opening andthe idly tosecure the center pole piece '502 to the bottom plate 50L At'its upperend the center pole piece 502 is provided with a portion 502!) -ofreduced -diameter, which extends within a centrally disposed opening503a formed in the top plate 503. As best shown in Figf of the drawings,the diameter of the opening 503a is slightly larger than the diameter ofthe upper portion 50% of the pole" piece 502. The upper end of thecenter pole piece is concentrically disposed gap is formed between theadjacentside walls The of three clamping screws 50 iv which extendthrough openings provided in the bottom plate 50! and are threaded intotapped holes drilled in the top plate 503. These screws serve rigidly toclamp the annular permanent magnet 500 be- .tween the top and bottomplates 503 and 5M.

The moving system of the translating device comprises a cone-shapeddiaphragm 505 which is formed of pressed fiber and is carried by asupporting structure which comprises a bottom ring 506, six spacingcollars 501, a pair of clamp- Fig. 8 arrangement"is used. By virtue ofthis frequencies is enhanced.

ing ring 508 and, 509 two paper, washers 5l0 and 5H, and six assemblyscrews 5l2. The

character of the diaphragm depends "upon whether thefldevice is to beused as a transmitting or receiving" element. If the device is to be.used as a'tran'smitter, the diaphragm is of one-piece'construction andis provided with an outerhatannular ring portion 505a which extendsbetween and is cemented to the paper "washers i510 and 5H and isuniformly clamped around its periphery between these washersby the twoclamping'rings508 and 509. If the device is to be used as a receivingelement, the diaphragm construction illustrated in Fig. 8 of thedrawings is usedQ In this construction only the bulbous portion of thediaphragm 505 r is constructed of molded or pressed fiber, the peripheryof this element being secured to an annular ring 505b' formed oftreated'silk which is disposed between and cemented'to the paper Washers510 and'5l I. The purpose of this ar- 'rangement is to permitsubstantiallyfree vibration of the diaphragm element 505, particularlyat the low -frequencies'of the operating frequency range, whereby theresponse of the device at these With the one-piece diaphragmarrangement, on the other hand, the diaphragmis considerably stifferand, accordingly, the 1 response thereof, particularly at lowfrequencies, is substantially. less than whenthe increasedstifinesaslowffreqdenoy noise components of sound rare prevented fromproducin any. substantial response. of -the device. I l 5.

A moving coil 5| 3.:isi'cemented to the-.- l0we bulbous portion "'ofthediaphragm-5505 and is concentrically disposed within they air gap.de-

fined by the upper portion 502b ofthe,poleipiece F 502 and the opening503a in the t'opplate..503.

Theterminal-ends of, this coil may be electrically connected'to lead-inwires in any desired manner although, preferably, the connections-aremade at anchor posts which extend through and -aresecured to thelowerbottom portionof the diaphragm505; 'In order to preserve the correctlateral and .axial spacing of the: moving coil 513 in the air gap of themagnetic circuit,

and to provideiadditional support for the diawithin the opening 503a,whereby an-annular air sides of the extended portion 502a is used rig-,

phragm 505, arstring supporting arrangement is provided. Thisarrangement comprises-three connected strings 514a, 5| 4b and 5l4c whichLextend through equiangularly spaced openings cut ,through the lowerportion of the diaphragm member 505, and are joinedtogethenas indicatedat-5l4d ,.within the dished cavity of this member; The free ends of thethree strings are respectively anchored by clamping the same to "thelugs 5l5a,-5|5b and 5l5c which extend radially inward-and areturnedupward from the supporting ring 506. Small clamping plates 5l5a,

, ingcollars 501 are used, to clamp the free ends 5l6b and 5I6c disposedbeneath shortened spacof the strings 5l4a,- 514D and -5I4c against thelugs 5|.5a, 5 I5b and 5l5c. Preferably the three strings 5l4a, 5M!) and5M0 are formed of silk fishline or the ;like,. and it will be;understood {that byappropriately tensioning the three stringstherequired support for the moving coil 5l3 may 1' be obtained: 1 1

l I In order to 'seal thejannular cavity, formed be-j 7o tween thespacedlapart annular permanent magnet 500 and the polevpiece 502 fromtheacoustical: cavity 5| 1 adjacent the lower side of the' diaphragm 505,thereby; to-enhance the damping Y of the. diaphragm and thus produce amore uniform response thereof over the entire operating frequency range,a sealing: ring SIB is provided which snugly encloses the-upper endportion 5112b of the center pole piece 502,'and isclamped to theunderside of the top plate 503-by means of assembly screws 519; 7,formed of rubben-molded Bakelite. or other insulating material havingnon-magnetic properties.

In the assembly of the translating device the magnetic circuit structureis built by first securing the sealing ring -18 to the underside of thetop plate 503 and then setting the assembly screws 5M'to clamp theannular permanent magnet 500 between the top and bottom plates 503 and501. After the magnetic circuitstructure is thus-assembled the portionofthe sealing ring 5l8 facing outward through the air gap of the structuremay be painted with shellac or other sealingcompound, to provide anacoustical seal which blocks oi the cavity of the magneticcircuitstructure-from the cavity'5l'l adjacent the underside of thediaphragm. 505. The'diaphragm assemblyis prepared by cementing thetwopaper washers 5H! and 5H to'the top and bottom'surfaces of thefiatannular portion 505a. ofthe diaphragm 505, assuming that atransmittingelement is being constructed,- and by cementing the movingcoil 513' to the lowercentral portion of the diaphragm member; If thedevice is to be used as a loudspeakingleceiver element, the. diaphragmstructure illustrated-inns. 8 of the drawings is constructed by firstcementing the'paper washers 510 and 5| lto the opposite peripheralsurfaces of'the silkringillib; after which the inner upper edgcisurfacesof the silk-ring are cemented to thlitidtsid of the'diaphragm505.

Following the construction of: the: diaphragm structure, the threestringsSllw, 51th and 5l4c are drawn through the angularly spacedopenings provided in the based the diaphragm 505. After. this operation:is performed, thesupportmg rin'g 'slls; the clamping rings 508' and 509and the assembled diaphragm structure may be stacked on the top plate503, and the assembly screws 5|! may successively be inserted throughthe registering opening provided in the stack clamping-rings anddiaphragm structure, the spacing collars 501', and the supporting ring506. Initially the screws which extend through the clamping elements 516are not tightened, but the.

alternate assembly screws which extend only through alternate spacingcollars 501' are tightened suflicient'ly to provide a semi-rigidassembly;

Following this operation the ends of the strings 5l4a', 5H!) and i l lc'may be threaded between the clampingelements 516a, 51Gb and-515i: and

their associated'lugg 5l5a, 5152i and'5l'5c, and properly tensione'd toprovide the required lateral and axial support'for the diaphragm 505.

Thereafter the assembly screws 512 may be tightened securely'to clampthe ends of the strings 514a, 5| 4b and'SIlc between the clampingelements 5|6a, 5l6b' and H50 and their respective associated lugs 5l5a,'5l5b and 5I5c. ,Thus the structure of the translating device iscompleted.

As pointed out above, each transmitting and receiving-element, asconstructed in the manner just described, is preferably embeddedina-block of acoustical insulating material to prevent the This ringispreferably tion, and more specifically may be of any configurationillustrated inFigs. 4a, 4b, 4c, 4d and 4e of the drawings, is preferablypreformed to the through. It has been found that the number therein, maybe provided.

of coats or the thickness of the layer of lacquer on the outsidesurfaces of the molded block determined to some extent, the acousticalpermeability of the structure. Hence, by appropriately controlling thedepth of the lacquer layer,

the acoustical characteristics of the sound transacoustical material 520may be either round or 25- square in horizontal cross section, and isprovided with an opening which snugly receives the side walls of thecompleted. transmitting or re-- ceiving element. If the unit is tooperate as a nondirectional device, the assembly is completed by closingthe opening in which the transmitting or receiving element is insertedwith a silk screen 523 having approximately'the characteristic impedanceof air, over which is placed a. thin sheet of aluminum 524 having anumber of holesoi appropriate size cut therethrough for the purpose oftransmitting sound. to or. from the diaphragm of the enclosed element. Aclamping ring and assembly screws, which are passed through registeringopenings in the clamping ring, the aluminum plate 524 and the moldedblock of insulating'material, may be used to hold the aluminum plate 524and the silk screen 523 in assembled relationship on the block ofinsulating material.

If directional characteristics are to be imparted to the device; anadditional block of molded acoustical insulating material, constructedinaccordance with the method briefly outlined above, and having soundtransmitting passages of appropriate lengths and dimensions preformed Asimple embodiment of-an acoustical insulating block provided with suchpassages is illustrated in Fig. 90f the drawings as being adapted foruse in conjunction with theunitillustrated in Fig. 5 of thedrawings.-More specifically, the Fig. 9 arrangement comprises a block ofacoustical insulating material 521 which is provided with a number ofsound transmitting passages 522 molded or out there- "through along theaxis of sound transmission to and from the diaphragm of the associatedtransmitting or receiving element. These passages are of the correctdimensions, 1. e., diameters and lengths, to provide for maximum soundenergy transfer between the atmosphere and the cavity immediatelyadjacent the outer surface of the V diaphragm. The block of acousticalinsulating transmission of sound waves and mechanical sulatingmaterial'52ll. This block oiinsulating material, whichmay be oithedesired-configuramaterial 52l may be secured to the block of insulatingmaterial 520 to clamp the aluminum sheet 524 and the silk screen 523 tothe top surface of the block 520 by means of assembly screws passedthrough registering openings in the-two insulating blocks.

1 From the foregoing explanation it willbe understood that atransmitting and a receiving eletively. It will also be understood thatin the unit as thus formed the conductivecoupling between the twoelements is reduced to a minimum by virtue of the vibration absorbingcharacteristics of the acoustical block. Moreover, the small effectivediameter of the loudspeaker element, as shown in Fig.5 of the drawings,contributes materially to the acoustical attenuation between thetransmitting and receiving elements.v This at tenuation may also beenhanced by utilizing the sound directing arrangements illustrated inFigs.

4b, 4c, 4d and 4c of the drawings in the construction of the block ofinsulating material, or by using either of the arrangements shown inFig. 4a

or 4c of the drawings. In short, the construction of the transmittingand receiving elements aswell as the physical arrangement of theseelements and the manner in which the elements are mounted, allcontribute to the reduction in acous-.

tical coupling between theseelements as provided at each station of thesystem. This means that the transmitting and receiving amplifiers ateach of the two connected stations l0 and II, as well as the channelamplifiers in the central repeater i2, may be operated with normal gainsettings which are substantially greater than the permissible gainsettings when-conventional microphone and loudspeaking arrangements areused.

In considering the operation of the system, it

may be assumed that the signal amplifiers and the control network, asprovided at each of the two stations l0 and l I, are active, and thatthe central repeater I2 is conditioned for operation. In this regard itwill be noted that to condition the circuit equipment provided at thewest station II for operation, the switch 338 is manually operated toits closed-circuit position wherein alternating current is delivered tothe parallelconnected windings 355 and331 of the power transformers 354and 336. With these transformers energized, low voltage alternatingcurrent is delivered to the cathodes of each of the tubes" 343', 322,326, 330 and 308 over obvious circuits.

'With the rectifier tube 342 in operation, current alternately traversesthe two anodes of the tube 342 during alternate half-cycles of thevoltage developed across the high voltage secondary winding 339, and ispassed through each of the two.

resistors 343 and 349 in the same direction. The alternating componentsof the voltage thus developed across the resistor 349 are minimizedthrough operation of the filter network comprising the choke coil 346and the shunt-connected condensers 34! and 348. The direct voltagedeveloped across the section 343a of the resistor '343 is negativelyapplied to the control electrode of the transmitting amplifier tube 306'to deter- 1 mine the operating point on the characteristic of this tube.Similarly, the voltage drop across the resistor section 3431) isnegatively applied to the control electrode 33017 of the tube 330 todetermine the normal space current flow through this tube.

Initially, the voltage dividing resistor 305 is so adjusted that themaximum signal output from the amplifier 300 consistent with stabilityof the station circuit is obtained" when heating current 5 of ratedvalue traverses thecathode 3060 ofthe tube 306. Similarly, the voltagedividing resistor 30'! is initially adjusted so that the maximum signaloutput from the amplifier 30! consistent with stability of the stationcircuit is obtained when the amplifier 30| is operating with normalsignal current gain therethrough. In this regard it will be apparentthat the stability of the system is determined by the combined signalcurrent gains through the two amplifiers 300 and 30! and, hence, thegain setting of each amplifieris dependent upon the setting of the otheramplifier. Assuming that the On-Off switch 36! oocupies the positionillustrated in the drawings, the signal current output from thereceiving amplifier 30| is in part determined by the setting of theadjustable voltage dividing resistor 30! and in part by the resistanceof the shunt connected space current path of the control tube 330. The

' resistance of the space current path of the tube 330 depends, in turn,upon the negative bias voltage impressed across the input electrodes3301) and 3300 thereof. Normally this voltage is equal to the sum of thevoltage drop across the resistor section 3432; and the voltage dropacross the cathode biasing resistor '352. The total magnitude of thisbias voltage is normally oi the order of 23 volts. With the two tubes308 and 330 thus conditioned for operation, the current traversing theparallel-connected space current paths thereof divides between theresistor 352 and the cathode 3060 of the transmitting amplifier tube306. The

value of the resistor 352 may be so'chosen that rated current normallytraverses the cathode 3060.

With the gain settings of the two amplifiers- 300 and 30l determined inthe above-described manner, the circuit constantsof the station circuitare so chosen that the signal current gains through the two amplifiers300 and 30! are substantially equal. Stability of the station circuitmay be then obtained during installation of the system by adjusting thevoltage dividers 305 and 301 until any tendency for the station circuitto oscillate is eliminated,

With the central repeater l2 conditioned for operation, but inactive, nosignal currents are transmitted through either of the two amplifiersections of the duplex diode tube 23!, andhence no bias voltage isdeveloped across the resistor 234. Accordingly the space current flowthrough the tubes 232 and 233 and their respective associated resistors236a and 23611 is determined by thesettingof the adjustable cathodebiasing resistors 235a and 235?). Each of these resistors is initiallyadjusted so that the space current flow through the associated tubedevelops a voltage of approximately [2 volts across the associatedresistor 236a or 236b.- 'The voltage across the resistor 236a isnegatively applied to the theresistor 23Gb is negatively applied to thecontrol electrode 2|2b of the east-west amplifier tube 2i'2. I v

Assuming that the microphone-loudspeaker unit illustrated in Fig. 4a ofthe drawings is utilized in conjunction with the circuit equipmentillustrated in Fig. 3 of the drawings and that a user of'the stationequipment speaks into the microphone 40!, the signal voltagedeveloped inthe moving coil of this microphone is transmitted portion of thisvoltage appearing across the lower nal current amplifier 105.

resistor 234.

portion of the voltage dividing resistor 305is impressed upon the inputelectrodes 306D and 306a of the transmitting amplifier tube 306. Thesignal currents as amplified by the tube 306 are transmitted through thecoupled windings of the 201D and 201C of the west east amplifier tube201 in parallel with the primary winding of the coupling transformer221. The signal currents as amplified by the tube 201 are transmittedthrough the coupling transformer 203 and the coupled windings of thehybrid system 202 to the line 13. From this point the signal currentsare transmitted over the line 13 and through the hybrid system 102 tothe input circuit of the sig- These currents as amplified by theamplifier 105 are transmitted to the moving coil of the loudspeaker 101for ;-reproduction.

The signal voltage as impressed between the input electrodes 223*?) and2230 of the amplifier tube 223 through the coupling transformer 221 andthe voltage divided 222, is amplified-through this tube and impressedthrough the coupling transformer 224 across the anode 231a and thecathode 231c of the duplex diode 231. More specifically, the voltageappearing across the secondary winding of the transformer 224 causes apulsating direct current to traverse the resistor 234; in a directionwhich may be traced as extending from the upper terminal of this windingby way of the resistor 230, the anode 231a, the space current pathbetween this anode and the cathode 2310 and the resistor 234 to thelower terminal of the indicated transformer winding. This pulsatingcurrent is smoothed through the action of the condenser 236 which shuntsthe Thevoltage thus developed across the resistor section 234a isnegatively applied to -the control electrode of the tube 232 to decreasethe space current-flow through this tube and thus lower the voltage dropacross the biasing resistor 230a. When this voltage drop is' decreasedthe negative bias on the control electrode 2011; of the active west-eastsignal amplifier tube 201 is lowered to increase the amplificationfactor of this tube. At the same time that the gain through the activewest-east channel 200 of the repeater 12 is thus increased, the gain ofthe inactive east-west channel 201 is correspond- -ingly decreased. Thusit will be noted that the voltage drop across the resistor section 23%is positively applied to the control electrode of the tube 233, wherebythe space currentfiow through this tube is increased. As a result thevoltage drop across the biasing resistor 23% is-increased. Since thisvoltage is negatively applied to the control electrode 212!) of the tube212, it will be understood that a corresponding decrease inthe signalcurrent gain through'the amplifier tube 212 occurs. 1 From the aboveexplanation it will be understood that the increase in signal currentgain through the active west-east transmission chan- 75 nel .200 isaccompanied by a corresponding decrease in the gain through the inactiveeast-west transmission channel 201 of the central repeater 12. Thus thestability of the repeater network is preserved. In this regard it willbe noted that the network includes a substantially closed signal trolamplifier tube 221.

current circuit which comprises the two channels 200 and 201 and thecoupling paths through the two hybrid systems 202 and 203, and that ifthe two amplifier tubes 20! and 212 are both operated at the high gainsettings required during signal current transmission, the totalgainaround the closed circuit may substantially exceed the electricallosses of the circuit. When this condition prevails the circuit networkis highly unstable and an oscillatory condition may be set up thereineven when signal currents of relatively small amplitudes are transmittedtherethrough. Due to the action of the control network 220, however, thetotal gain around the closed circuit is maintained below thepredetermined value at which the circuit is stable by concurrentlyincreasing the gain of the channel in use and decreasing the gain of theinactive channel. Accordingly, the stability of the repeater ispreserved even though signal currents of substantial magnitude aretransmitted through the repeater. Each time signal current transmissionover the line 14 is terminated to arrest the signal input to the controlnetwork 220, the bias voltages across the two resistor sections 234a and23411 are reduced to zero, whereby the normal bias voltages across thetwo biasing resistors 235a and 23Gb are restored. The signal currentgain throug I .the amplifier tube 201 is thus decreased to normal andthe signal current gain through the amplifier tube 212 is increased toits normal value.

The rate at which the bias voltages across the values of this networkare proportioned to provide the necessary hangover period at each signalcurrent break-off point, in order to prevent syllable clippin whichwould otherwise occur.

When sound waves are transmitted to the microphone provided at the eaststation 10, corresponding signal currents are developed in the outputcircuit of the signal current amplifier 104 and are transmitted throughthe hybrid system 102, over the line 13 and through the hybrid system202 to the parallel-connected input circuits of the channel amplifiertube 212 and the con- These signal currents as amplified by the channelamplifier 201 are transmitted through the coupling transformer213 andthe hybrid system 203 to the line 14. From this point they aretransmitted over the line 14 and through the hybrid system 302 to theinput circuit of the receiving amplifier 301 provided at the weststation 1 1. After being amplified by the receiving amplifier tube 308,the signal currents are transmitted through the coupling transformer 309and over the cord conductors 431 to the moving coil of the loudspeaker402 for reproduction.

The signal voltage as impressed between the inputelectrodes of theamplifier tube 22'! through the coupling transformer 225 and the voltagedivider 22B is amplified by this tube and impressed through the couplingtransformer 22B across the anode 231d and the cathode 231e of the duplexdiode 231. More specifically, the voltage appearing across the secondarywinding of the transformer 228 causes a pulsating direct current totraverse the resistor 234 in a direction whichmay be traced as extendingfrom the upper terminal of this winding by way of the resistor 229, the

trode of the tube 232 to increase the space current flow through thistube and thus increase the voltage drop across the biasing resistor233a. When this voltage drop is increased the negative bias on thecontrol electrode 2371) of the'inactive west-east signal amplifier tube201 is increased to decrease the amplification factor of this tube. Atthe same time that the gain through the mac- 'tive west-east channel 200of the repeater i2 is thus decreased, the gain of the active east-westchannel 20! is correspondingly increased. Thus it will be noted that thevoltage drop across the resistor section 23413 is negatively applied tothe control electrode of the tube 233 so that the space current fiowthrough this tube is decreased. As

a result, the voltage drop across thebiasing resistor 23612 isdecreased. Since this voltage is negatively applied to the controlelectrode 2l2b of the tube 2l2, it willbe understood that a decrease inthe magnitude of this voltage produces a corresponding increase in thesignal cur-3.,

rent gainthrough the amplifier tube 2 [2. Thus it will be seen that whensignal currents are transmitted in the east-west direction from thestation I to thestation Ii,thesignal current gain through the activechannel is increased the required amount for satisfactory reproductionpfthe signal currents by the loudspeaker 402 in use, and this signalcurrent gain is accompanied by a corresponding decrease in the gain ofthe inactive west-east channel 200, whereby the stability of;

the central repeater I2 is preserved. It will also be understood fromthe above explanation that each timesignal current transmission over theline l3 from the east station I0 is terminated to arrest the signalinput to the control network 220,,

the bias voltages across the two resistor sections 234a and 2341) arereducedto zero after a short time interval which is determined by thetime constant of the shunt circuit including the resistor 234 and thecondenser 236. Accordingly, the bias voltages across the two biasingresistors 236a and 23Gb are again equalized at their respective normalvalues, whereby the signal current gainthroughthe amplifier tube 201 isincreased to normal and the signal current gain through the amplifiertube 2l2 is decreased to its normal value.

Referring now more particularly to the method of controlling the network315 provided at the west station I I, it will be noted that a' portionof the signal voltage appearing across the output circuit of thereceiving amplifier tube 308 is impressed across the input electrodes 322b'and 3220 of the amplifier tube 322 through the contacts of the switch36! and the frequency discriminating network 3I6. Aspointed out above,this network is designed to favor signal currents having frequencies inthe band ranging fromapproximately 300 cycles persecond to approximately1200 .cycles per second, In thlsregardit will be understood that the'ratio of the signal voltages across the two'resistors 3!!) and SM isdetermined by the reactanceof the condenser 320. At relatively lowsignal voltage frequencies the reactance of this condenser isrelativelyhigh so that a relatively large proportion of the available voltage dropacross the two series-connected resistors 3 i9 and 32I appears acrossthe resistor 32L On the other hand, as the signal current frequencyincreases, a' corresponding decrease occurs in the reactance of thecondenser 320, so that an increasing percentage' of the available signalvoltage appears across the resistor 3E9. Thus by properly proportioningthe constants of the network3l6, this network may be operated to favorsignal currents of the frequencies within the band indicated. The reasonfor providing a discriminating network 3H3 which favorssignal currentfrequencies within the low end of the-operating range-is to make thecontrol network 3l5 primarily responsive tothe band of frequencies atwhich the major portion of thevoice'current energy is produced. Thus thepredominant portion of the frequencies used in ordinary speech lieswithin the band ranging from 300 to approximately 1200cycles per second.i

The signal voltage as developed across the resistor 32l 'is amplified bythe tube 322, and the amplified signal currents cause correspondinginducedvoltages in the secondary winding of the coupling transformer324, which voltages are applied across the anode and cathode of therectifying tube 326. This tube is biased by means of the battery 325 toa point .on its operating charthe amplifier tube 308 exceeds apredetermined value, determined by the voltage of the biasing value.Thus, so long as the signal voltage across the output circuitof thereceiving amplifier tube 308 does not exceed a predetermined value, thebiasing battery 325. prevents the control network .3l5from changing thegain of either of the .two

amplifiers 30| or .302. When, however, the signal voltage appearingacross the output circuit of battery 325,the signal voltageacross thesecondary winding of the transformer 324 becomes sufiicientto causecurrent to traverse the space current path of the tube 326. Thiscurrenttraverses the biasing resistor 328, is smoothed by the condenser329, and flows in a direction such that the resulting direct voltageacross the resistor 328 is additive with respect to the bias voltageacross the section 343b, of the resistor 343. Ac-

" cordingly the net bias voltage across the input electrodes 3302) and3300 of the, control tube 330 is increased to increase the resistance'ofthe space current path of this tube. Incidentto this increase in spacecurrent resistance of the tube 330, the shunting effect of the tube onthe output circuit of the receiving'amplifier tube 308 iscorrespondingly decreased to produce anincrease in the signal currentgain through the amplifier 30 I. This decrease in the shunting effect ofthe tube 330 is supplemented byan increase in the amplification, factorof the tube 308 occasioned by an increase in the voltage on the anode308a. Thus when the space current flow through the tube 330 isdecreasedthe voltage drop across the resistor .which the control circuit315 starts inversely to change the gains of the two amplifiers 300 and301, may be adjusted, through suitable adjustment of the voltage divider318, to vary the proportion of the available signal voltage which isimpressed between the input electrodes of the tube 322. This adjustmentis determined to a large extent by the 'normal gain settings of the twoamplifiers 3011 and 301. Hence, after the required signal current gainthrough the two tubes is once established, the voltagedivider 318may beadjusted to provide the minimum signal voltage input to the amplifiertube 322 consistent 1 with the required control of the station circuit.

It will also be understood from the above explanation that after thecontrol circuit 315 starts to operate, the extent to which the signalcurrent gains of the two amplifiers 300 and'301 are inversely changeddepends, within limits, upon the magnitude of the signal currentsincoming to the west station 11' overthe line 14. Preferably, thecircuit constants of the network 315 are so adjusted that the amount ofthe decrease gain of the-signal amplifier 3011 which occurs in responseto any given Change in the input signal intensity is just sufficient tomaintain the stability of the station circuit. -As indicated by theabove explanation, the gain control action realized through operation ofthe network 315 isnot effective until the-signal input to theloudspeaker 1132 exceeds a predetermined value. By virtue of thisarrangement, the'control network '3 1 is rendered substantiallynon-responsive to noise cur- 5 rents resulting from background noiseswhich may be'-acoustic'ally imposed'onthe system 'or to noisecurrentsresulting from electrical transients imposed on the system,which currents are usually of a low order of magnitude, -Since suchnoise currents'areusually' of low frequency, the couplin'g condenser31'! maybe chosen of such value as to prevent-any substantial'portion ofthere- "slllting voltage *across the output circuit'of the amplifiertube308 fromappearing'across the voltage dividing resistor 318. Thus,the response of the network 315 islimitedto speech or other actualsignal "current transmission over the -two connected lines'of the systemin'an east-west direction. Further, the constants of the station circuit are so chosen'that-the variation in gain'of *theamplifier300,-efiected through operation of the control network 31 5,are limited toa definite range which will include all cases of normalsigparent that amplified-signal voltages appearing across the outputcircuit of the tube 308 are not impressed upo-nthe frequencydiscriminating circuit 316. Thus the control network 315 is renderedcompletely inactive, with the result that current of a substantiallyconstant magnitude is caused to traverse the cathode 31360 of thetransmitting signal amplifier tube 306 regardless of the level of signalcurrent transmission to the station 11. While it will be understood thatthe specifications of the circuit provided at the station H may varyaccording to the design of a particular installation, the followingspecification of circuit constants for the station circuit of Fig. 3 isincluded by way of example as being satisfactory.

Tube 306 ;Commercia1 type 47 Tube 308 Commercial type lA5G Tube 322Commercia1 type 26 Tube 326 Commercial type 26 Tube 330 "Commercial type10 Tube 342 Commercial type 80 Condenser 363, 0.25 microfarad Condenser365 50.0 microfarads Condenser 317 0.001 microfarad 5 Condenser. 3200.002 microfarad Condenser 323 0.05 microfarad Condenser 327 8.0microfarads Condenser 331 0.25 microfarad Condenser 329 0.01 microfaradCondenser 345 8.0 microfarads Condenser 344--.; 8.0 microfaradsCondenser 348 16.0 microfarads Condenser 347 8.0 microiarads Condenser353-; 5.0 microfarads Condenser 360--... ..5.0 micro-farads Resistor 305r 250,000 ohms Resistor 364 500,000 ohms Resistor 366' n 500,000 ohms 4Resistor 307' "250,000 ohms Resistor 362 1250 ohms Resistor 31 8"250,000 ohms Resistor 319 100,000 ohms Resistor 321 500,000 Ohms--Resistor 328 5 00,000 ohms Resistor 343- 250 m Resistor 349 25,000 ohmsResistor 350 1 5,0'00 ohms Resistor 351 r 420 ohms .50 Resistor 352 ohmsResistor 359 1000 ohms Battery 325 "4.5 volts Although only one-controlnetwork 315, which responds to signalcurrents incoming tothe station 11to control the gains of the-amplifiers 3110 "and 381, has beenillustrated in the drawings, it will be understood that a second controlnetwork of like arrangem'entand responsive to signal currentstransmitted from'the microphone 401 "to the amplifier 3l10,'maybeprovided if necessary. In suchcase, the input and output circuits of thesecond control network will be bridged across'the output circuit'of theafiiplifier tube306 and-the space current traversing-thecontrol't'ube'oi the second networkwill determine the cathode heatingcurrentof the receivingam plifier tube. With such an arrangementtheseco'nd control circuit will function to-i'ncrease the gain ofthe-transmitting amplifier tube 30'6andto-decrease the '-.gain of thereceiving :a mplifier tube when sound.

waves-exceeding a predeterminedvalue aretransinitted to the microphone401.

The loudspeaking system-shown in the drawings of the present applicationis' claiined in the copendin'g application of Gabriel- Giannini,-'Serial No. 419,165, filed November 14,1941, now Patent No. 2,341,539,granted February 15,1944.

While one embodiment of the invention has been described, it will beunderstood that various modifications may be made therein withoutdeparting from the true spirit and scope of the invention.

Whatv is claimed is: f

1. In a transmission system, a pair of variable gain signal currentchannels'one of said channels including an amplifier tube having acathode, a circuit for energizing said cathode, means controlled bysignal currents traversing the other of said channels for changing thegain through said other channel in one sense and. for controlling thecurrent traversing said circuit to change the gain through said onechannel in the opposite sense, and means for preventing the operation ofsaid last-named means until the signal input to said other channelexceeds a,

predetermined value.

2. In a transmission system, a pair of signal current channels, one ofsaid channels including an amplifier tube having a cathode, a controltube having output electrodes defining a space current path which isbridged across the other of said channels, whereby the signal currentgain through said other channel is changed as the resistance of saidspace current path is varied, means for energizing said cathode with acurrent which varies in accordance with the resistance of said spacecurrent path, whereby the signal current gain through said one channelis changed as the resistance of said space current path is varied, andmeans controlled by signal currents traversing said other channel forcontrolling the resistance of said space current path.

3. In a transmission system, a pair of signal current channels, one ofsaid channels includtrol tube having output electrodes defining a spacecurrent path which is bridged across the other of said channels, wherebythe signal current gain through said other channel is varied directly inaccordance with the resistance of said space current path, means forenergizing said cathode with a current which varies inversely inaccordance with the resistance of said space current path,,whereby thesignal current gain through said one channel is varied inversely inaccordance with the resistance of said space current path, and means forcontrolling the resistance of said space current path directly inaccordance with the amplitude of the signal input to said other channel.

4. In a transmission system, a pair of signal current channels, one ofsaid channels including an amplifier tube having a cathode, a controltube having output electrodes defining a space current path which isbridged across the other of said channels, whereby the signal currentgain through said other channel is varied ing an amplifier tube having acathode, a con the operation of said last-named means until theamplitude of the signal input to said other channel exceeds apredetermined value.

5. In a transmission system, a pair of signal current channels, .one ofsaid channelsi including an amplifier tube having a cathode, a controltube having output electrodes defining a space current path, a resistorshunting said cathode and lbridged across the other of said channels .inseries with said space current path, whereby the signal current gainthrough said other channel, is varied directly in accordance with theresistance of said path and the current traversing said cathode isvaried inversely in accordance with the resistance of said space currentpath, and means for varying the resistance of said space current pathdirectly in, accordance with the amtube having output electrodesdefining a space current path, a resistor shunting said cathode andbridged across the other of said channels in series with said spacecurrent path, whereby with the resistance of said space current path,

means for varying the resistance of said space current path directly inaccordance with the amplitude of the signal input to said other channel,and means for preventing the operation of said last-named means untiltheamplitude of the signal input to said other channel exceeds apredetermined value.

'7. In a transmission system, a pair of signal current channels eachincluding an amplifier tube, the tube in one of said channels includinga cathode, a controLtube having output electrodes defining a spacecurrent path connected in parallel with the space current'path 0f theamplifier tube in the other of said channels, a resistor shunting saidcathode and connected in seriesfwith said parallel connected spacecurrent paths, whereby the signal current gain through said otherchannel is varied directly in accordance with the resistance of thespace current path of said control tube and the cunrent traversing saidcathode is varied inversely in accordance with the resistance of thespace current path of said control tube, and means for varying theresistance of the space current path of said control tube directly inaccordance with the amplitude of the signal input to said other channel.

8. In a transmission system, a pair of signal current channels eachincluding an amplifier tube, the tube in one of said channels includinga cathode, a control tube having output electrodes defining a spacecurrent path connected in parallel with the space current path of theamplifier tube in the other of said channels, a resistor shunting saidcathode and connected in series with said parallel connected spacecurrent paths, whereby the signal current gain through said otherchannel is varied directly in accordance with the resistance of thespace current path of said control tube and the current traversing saidcathode is varied inversely in accordance with the resistance of thespace current path of said control tube, mean for varying the resistanceof the space current path of said control tube directly in accordancewith the amplitude of the signal input to said other channel, and meansfor preventing the operation of last-named means until signal input tosaid other channel exceeds a'p'redetermined value.

10. In a transmission system, a pair of signal currentchannels eachincluding an amplifier tube having a cathode, a cathode resistorincluded in the space current path of one of said tubes and shunted bythe cathode of the other of said tubes. a variable resistance pathshunting the space current path of said one tube, and means controlledin accordance with the signal input to the other of said channels forcontrolling the resistance of said variable resistance path.

11. In a transmission system, a pair of signal current channels eachincluding an amplifier tube having a cathode, a cathode resistorincluded in the space current path of one of said tubes and shunted bythe cathode of the other of said tubes, a variable resistance pathshunting the space current path of said one tube, means controlled inaccordance with, the signal input to the other of said channels forvarying the resistance of said variable resistance path, and means forpreventing the operation of said last-named means until the signal inputto said other channel exceeds a predetermined value.

12. In a transmission system, a pair of variable gain signal currentchannels, a control net-' work in which the input and output circuitsare coupled to one ofsaid channels at the same point and including meanscontrolled from said point in accordance with the signal input to saidone channel vfor inversely varying the gains of said two channels, thegain of said one channel being controlled at said point, and meansincluded in said network for preventing the operation of said last-namedmeans until the signal input to said one channel exceeds a predeterminedvalue.

13. In a transmission system, a pair of signal current channels eachincluding an amplifier tube, the tube in one of said channels including:a cathode, a circuit for energizing said cathode, and a control networkin which the input and output circuits are coupled to the other of saidchannels at the same point following the tube in said other channel andincluding means controlled in accordance with the signal input to saidother channel for varying the signal current gain through said otherchannel and for varying the current traversing said circuit.

14. In a transmission system, a pair of signal current channels eachincluding an amplifier tube, the tube in one of said channels includinga cathode, a circuit for energizing said cathode, a control network inwhich th input and output circuits are coupled to the other of saidchannels at the same point following the tube in said other channel andincluding means controlled in accordance with the signal input to saidother channel for varying the signal current gain through said otherchannel and for varying the current traversing said circuit, and meansincluded in said network for preventing the operation of said last-namedmeans until the signal input to said other channel exceeds a predator.mined value.

GABREL M. GIANNINI. 'BEN EISENBERG.

